The problem is that the characteristics of certain electrodes may become modified during the manufacture of a cathode-ray tube, and may consequently modify certain characteristics of the tube.
When the gun is being sealed to the tube, heating by flames or the like melts the glass of the tube neck and the glass of the gun base in order to weld them together in a vacuum-tight fashion. Owing to this heating in the atmosphere, the parts of the gun close to the base heat up and therefore tend to become oxidized at the surface, especially in the case of the electrodes G1 and G2 (see FIG. 1). These electrodes, however, are subsequently bombarded by the electron beam of the gun during activation of the cathodes and during the emission measurements, if these are performed without scanning the screen, which causes dissociation of the surface oxides into metals and oxygen gas. Moreover, oxygen is a poison for the cathodes since it degrades their electron emission. One symptom is that the emission starts up again poorly after the cathode-ray tube has been stored for a few days or weeks.
Later, when pumping the cathode-ray tube, radiofrequency induction heating of the gun is carried out by means of an electromagnetic self-inductance with a view to degassing the gun. In this context, the metal parts of the gun are heated and therefore expand, respectively as a function of their temperature and the coefficient of thermal expansion of their material. Mechanical stresses are created because the expansions are not balanced between the parts, which are rigidly connected to two sintered glass bars VF1 and VF2 constituting the framework of the gun. The hottest parts of the gun are in this case the electrodes G2 (heated to a temperature of about 750° C.), G3 (heated to a temperature of about 790° C.) and G1 (heated to a temperature of about 680° C.). The drawback of the mechanical stresses is a remanent deformation of certain parts of the gun, and in the worst case cracking or fracture of the two sintered glass bars VF1 and VF2 (especially if they experience mechanical stresses when the gun is being cooled after the end of the RF heating).
During operation of the cathode-ray tube at startup of the cathode-ray tube, expansions are subsequently caused by the heating filaments and increase up to the steady-state regime corresponding to the time at which the filaments and the cathodes have reached their rated temperatures (generally with 6.3 V across the terminals of the filaments). The most strongly. heated metal parts of the gun are the ones closest to the heating filaments and the cathodes, particularly the cathode supports, the electrode G1 and the electrode G2. In this context, the drawback of the mechanical stresses is an imbalance of the picture colors (color temperature change: CTC) due to differences between the red, green and blue beam currents, the CTC being caused by the problem of non-remanent deformation at startup of the cathode-ray tube.
Furthermore, the cost of the gun depends in particular on the cost of the materials constituting the parts of the gun. Alloys having low coefficients of thermal expansion, such as the metal alloys of the family FeNi (that is to say in which Fe and Ni make up more than 95% of the mass) and the metal alloys of the family FeNiCo (that is to say in which Fe, Ni and Co make up more than 95% of the mass) are more expensive than stainless steels.
Electron guns in which the electrodes are made of FeNi, and which for example have the characteristics summarised in the table below, are known:
Tube startup, 6.3 V beingRF induction heating ofappliedthe gunCoefficientCoeff.ExpansibleofExpansionofwidthexpansionatexpansionRFbetween theT°of thestartupT°of theexpansionglass barsstabilizedmaterial(μm)stabilizedmaterial(μm)SelectedUnitsmaterialmm° C.10−6/° C.μm° C.10−6/° C.μmG4 et seq.FeNi4215705.366007.668G3FeNi4815808.71079011.4135G2FeNi42151205.3107508.697G1FeNi42151805.3146808.082cathodeFeNi42153006.0275507.058supports
FIG. 2 represents a graph indicating the expansions of the electrodes G1 to G4 and of the cathode supports in such an electron gun during RF induction heating and at startup of the gun. It can be seen that such an electron gun exhibits expansions which are acceptable and, in particular, approximately uniform for the various electrodes in RF. The electrodes G1 and G2, however, are not resistant to the oxidation and present a strong risk of having poor electron emission.
Another type of electron gun, such as the Toshiba and Matsushita guns in particular, uses the material “Kovar” (FeNiCo alloy) for G1 and G2. This alloy has a low coefficient of thermal expansion but cannot withstand the oxidation as much a stainless steel, and it is more expensive.
It will be understood that there is no known electron gun in which the electrodes and the electrode supports are made of a material such that:                the electron gun is resistant to the emission problem caused by oxidation of the electrodes G1 and/or G2 when it is being sealed into the tube (mount-sealing),        there are no expansion problems detrimental to the working life of the gun,        the CTC (color temperature change) is stable and acceptable.        
A conventional solution to the problem of oxidation is to use conventional stainless steel from the family of austenitic steels, such as the Type 305 steel whose UNS designation is S30500, for the electrodes G1 and G2. In this case, however, the electron gun will not be resistant to the problem of thermomechanically induced remanent deformation caused by heating in the course of the radiofrequency induction (RF heating) for pumping. Furthermore, the gun then has a mediocre “CTC” (color temperature change).
When wishing to make the gun more resistant to the problem of thermomechanically induced remanent deformation caused by heating in the course of the radiofrequency induction (RF heating) for pumping, the known solution is to use alloys having lower coefficients of thermal expansion for the electrodes G1, G2 and G3, and more specifically metal alloys whose coefficient of expansion between 20° C. and 300° C. lies between 3×10−6/° C. and 7×10−6/° C.
These metal alloys, however, such as those of the family FeNi (that is to say in which Fe and Ni make up more than 95% of the mass) and of the family FeNiCo (that is to say in which Fe, Ni and Co make up more than 95% of the mass) are more expensive than stainless steels.
When wishing to provide the gun with an acceptable “CTC” (color temperature change), for example as described in U.S. Pat. No. 4,492,894, an electron gun may be provided in which the materials of the successive electrodes of the gun are selected so as to balance the expansions of these electrodes in the steady-state regime corresponding to the time at which the filaments and the cathodes have reached their rated temperatures (generally with 6.3 V across the terminals of the filaments). The hottest electrodes will therefore have the lowest coefficients of expansion.
Then, however, the electrode G3 will have a higher coefficient of thermal expansion than G2 even though G3 is already hotter then G2, and the electrode G2 will have a higher coefficient of thermal expansion then G1 even though G2 is already hotter then G1. The electron gun will not therefore be resistant to the problem of thermomechanically induced remanent deformation caused by heating in the course of the radiofrequency induction (RF heating) for pumping.
U.S. Pat. No. 4,468,588 addresses the CTC problem. This patent describes a solution in which the cathode supports minimize the deformations of the electrode G1 with respect to the cathodes. This document, however, does not resolve the emission problem caused by oxidation of the electrodes G1 and/or G2 when it is being sealed into the tube (mount-sealing), nor the problem of making the gun more resistant to the thermomechanically induced remanent deformations caused by heating in the course of the radiofrequency induction (RF heating) carried out when pumping the cathode-ray tube.